I. Fibroblast growth factor (FGF) receptor signaling in development and human skeletal diseases FGF receptors constitute a family of four membrane-spanning tyrosine kinases (FGFR1-4) which serve as high affinity receptors for at least fifteen growth factors (FGF1-15). FGF/FGFR signals may have many important biological functions, including developmental induction and differentiation, wound healing, tumor angiogenesis, cell growth and migration, as well as neuronal differentiation and survival. Moreover, it has been recently demonstrated that alterations in FGFRs are responsible for at least nine human inherited diseases. All of these diseases are caused by single amino acid mutations and exhibit extensive craniofacial or axial and appendicular bone abnormalities. To study functions of FGFs/FGFRs signals in development and to gain insight into mechanism(s) underlying these inherited diseases, mice that carry mutations on each receptor have been created through gene targeting. Analyses of these mutant mice revealed that FGFR-1 is essential for axial organization during gastrulation, that FGFR-2 plays a role in placenta formation and limb initiation, that FGFR-3 is a negative regulator in bone growth and development, and that both FGFR-3 and FGFR-4 function cooperately to control postnatal lung development. Point mutations in different domains of FGFR-3 have been reported to result in achondroplasia (ACH) and thanatophoric dysplasia (TD), the most common forms of dwarfism and neonatal lethal skeletal dysplasia, respectively. In several respects, the phenotypes caused by loss of FGFR-3 in mice are opposite to those of ACH and TD patients, suggesting that the human mutations result in a gain of function of the receptor. To create animal models for the human diseases, ACH and TD type mutations have been introduced into mouse genome. The resulting mice exhibit phenotypes that mimic the human diseases. We are currently in a process to characterize these mouse models. Gene therapy will be carried out shortly. II. BRCA1-flox mice: an animal model for breast cancer Germline mutations of the Brca1 gene are responsible for most cases of familial breast and ovarian cancers. Functional interaction of BRCA1 with other tumor suppressor genes, such as p53 and p21 in regards of tumorigenesis are one of our major interesting. We found that mouse embryos deficient for BRCA1 were developmentally retarded at early embryonic development and hypersensitive to g-irradiation, suggesting a failure in DNA damage repair. However, massive chromosomal abnormalities were only observed when a p53-/- background was introduced. Thus, a p53 dependent cell cycle checkpoint arrests the mutant embryos and prevents the accumulation of damaged DNA. Brca111-/- fibroblasts were not viable, nor were Brca111-/-:p53-/- fibroblasts. However, proliferative foci arose from Brca111-/- :p53-/- cells, probably due to additional mutations that are a consequence of the accumulating DNA damage. We believe that the increased incidence of such additional mutations accounts for the tumorigenesis associated with Brca1 mutations (10). To overcome lethal effect of the BRCA1 deficiency, Brca1-flox mice that carry loxP sites flanking exon 10 of Brca1 gene have been created. The Brca1-flox mice have been crossed into MMTV-cre and WAP-cre mice respectively. Mice carrying deletion of BRCA1 in mammary gland have been produced and monitored for tumor formation. III. Core facility for knockout mice A core facility has been established to generate knockout mice for NIDDK investigators. Mutant mice for a variety of gene knockouts have been produced in the facility.